The present invention relates generally to uninterruptible power supply (UPS) systems, and more particularly, to two or more UPS systems connected and operated in parallel.
A UPS system is used in applications that require continuity of power such that when the main power source fails, the UPS system provides power from a reserve energy storage system, typically in the form of a battery. The UPS system monitors the main power source and controls the UPS components to provide continuous power to the critical load.
To provide further continuity of power, multiple UPS modules may be paralleled with other UPS modules to form a redundant UPS system with a given multiplicity. This adds redundancy in that any UPS module may be disconnected from the critical load for service or automatically by the UPS to isolate a faulty module. The remaining module(s) still provide power to the critical load. Typically when UPS modules are paralleled, some module-to-module wiring is added to facilitate the sharing of power and the connection or disconnection of modules.
When a module needs be removed from the critical load, either manually for service or automatically due to a fault, it is vital that the module detect if at least one other module is able to support the critical load. If there is no other module(s) to support the critical load, then the module must transfer the critical load to its bypass source. Detecting the presence of redundant UPS modules could easily be done with the module-to-module wiring; however, this creates a dependency on that connection. It is thus desirable to detect the presence of other UPS modules using only local measurements.
Known systems detect the presence of other UPS modules using only local measurements by simply turning off or disconnecting the UPS source that becomes faulty and/or needs to be taken off line, then letting the UPS module monitor the output voltage. If the output voltage remains qualified, it can be assumed there are other UPS modules present supporting the load. If the output voltage becomes disqualified, then no other UPS module is present to support the load, and the UPS module may transfer the critical load to its bypass source. This approach however creates a large distortion or fluctuation on the output voltage when other modules are not present since depending on the load present, the effect of turning the source off or disconnecting it may be unpredictable.
The present application addresses these shortcomings associated with the prior art.